1. Field of the Invention
The present invention relates to a method and apparatus for an optical switch and routing based on quantum coherence, specifically using slow light phenomenon and nondegenerate four-wave mixing processes.
2. Description of the Related Art
An optical router is a switching device applied to convert an optical signal into another one at different propagation direction with either the same frequency basis or not. The optical router is a subcategory of an optical switch that is in general used to drop, add, multiplex, or convert an optical signal into another one. In fiber-optic communications, as more data traffic increases, more information bandwidth is needed. In this case a wider bandwidth optical switch is obviously demanded. However preparation of wide bandwidth optical switch is not simply easy because of technical limitations in switching devices such as an inherently limited optical switching time and slow electronics. Thus, more often it is required that the data traffic in fiber-optic communication lines need to be temporally holt for some data processing purposes. Obviously, an optical buffer memory becomes an essential component to an optical data processing unit. In the optical switching area, several switching methods have been introduced. Of them are semiconductor-optical-amplifier (SOA)-based optical switch, LiNbO3 optical switch, and semiconductor intersubband-utilized optical switch. The SOA-based optical switch and LiNbO3 optical switch are utilizing refractive index change induced by electric current or voltage. In the conventional optical switching technologies, the time needed for the refractive index change has been a constraint of the optical switching devices. Thus 10˜100 GHz optical switching has been an upper limit. Based on these optical switching technologies, faster switching relates to shorter pulse length. Therefore, the relation between the switching time and the bandwidth is inversely proportional to each other.
On the other hand, it is well known that resonant two-color electromagnetic fields can induce refractive index change in a nonlinear optical medium composed of three energy levels or more. In a three-level optical system composed of two-closely spaced ground states and an excited state, or two-closely spaced excited states and a ground state, or arbitrarily spaced states, the refractive index change can result in not only absorption cancellation at line center but also two-photon coherence excitation between two closely spaced levels. This phenomenon is called dark is resonance or electromagnetically induced transparency (EIT) in the context of optically dense medium. Because EIT modifies the absorption spectrum of the medium, the medium's dispersion must be also modified via Kramers Kronig relations. Thus, the group velocity of a traveling light pulse can be controlled to be slowed down. This is so called a slow light phenomenon. Recently the slow light phenomenon has been observed in cold atoms, defected solids, optical fibers, and semiconductors.
In the case of dark resonance or EIT, the time needed for the refractive index change is, however, not limited by the carriers' lifetime or population relaxation time, but limited by the phase decay time. Generally, the phase decay time is faster than the carrier's lifetime at least twice in most atomic gases and hundreds times in most ion-doped crystals such as Pr3+-doped Y2SiO5. The two-photon coherence excitation on the closely spaced ground states can also be optically detected via nondegenerate four-wave mixing processes. The optical intensity of the nondegenerate four-wave mixing signals can be stronger than that of the original input laser. This signal amplification in the nondegenerate four-wave mixing processes based on EIT has already been demonstrated experimentally.
Even optical component is eligible for 100 GHz switching, electronic counterparts are not. Thus, speed constraints in an optoelectronic device lies on electronic part. If one can slow down optical switching speed without affecting overall switching bandwidth across the switching/routing device, then slow electronics can come up with the fast optical counterpart. This is the main motivation of the present invention and will be explained in detail below.